Biosynthesis of nanocrystalline silver chloride with high antibacterial activity using bacterial extracts

Abstract

The traditional synthesis of nano silver chloride involves chemical precipitation or physical methods. Due to the hazardous substances generated during the synthesis, it can cause environmental pollution. In this study, a green and facile method is described to biosynthesize nano-silver chloride with excellent antibacterial activity via in situ reduction of Ag ⁺ using the extract of Bacillus thuringiensis (Bt). Compared with previous reports, the minimal inhibition concentration (MIC) against Escherichia coli (E. coli) is 3.0 μg/mL, which is 2– to 800-fold higher than that of nano silver chloride. Subsequent in vitro studies involving agricultural bacteria such as Ralstonia solanacearum (R. solanacearum) revealed a 92.95% antibacterial rate when the concentration of nano-silver chloride was 2.0 μg/mL. Previous studies of antibacterial activity of nano-silver chloride focused more on the basic antibacterial properties without describing its antibacterial molecular mechanisms. The microscopic investigations and DNA damage experiments indicated that the nano-silver chloride adsorbed to the bacterial surface, leading to cell wall rupture, DNA damage, and cytoplasmic leakage. In addition, electron paramagnetic resonance (EPR) spectroscopy indicated the synthesis of reactive oxygen species (·OH, ·O²⁻ and ¹O₂) in the bacteria. Our study provides evidence supporting the use of nano-silver chloride as an antibacterial agent in agriculture, and theoretical insight into the antibacterial mechanism thereof.